The obvious lack of reproducible experiments is in our opinion one of the main reasons for the slow progress in parapsychology. One could maintain that during some experiments with talented subjects results have been obtained which proof the existence of psi beyond any reasonable doubt. However, experiments in which the influence of different variables are studied, are not expected to yield consistent results. Indeed it is not unusual that upon replication some of the effects observed in the earlier experiments are found to be reversed (significantly) in the replication experiment.

A method of obtaining reproducible results may be found in large scale experiments, i.e. many subjects and/or many trials. Recent experiments of Schmidt (1973) are examples of this 'large scale' direction. It is remarkable that in these experiments the interaction being studied is that between a living entity and a machine. It may be that such a system is most promising for getting reproducible results, as one half of the agent-percipient pair is a machine, the behaviour of which is known to a larger extent than that of a human being.

A theory of psi-phenomena proposed recently by Walker (1972) is the reason for having two types of trials in our experiment. In one type the subject tries to 'influence' random processes in real time (Psychokinesis) while in the other type the subject is acting upon processes that have already been recorded in the past (retroactive Psychokinesis). According to the theory the conscious observation of the random events by either the subject or the checker/experimenter (see also Feather & Brier, 1968) is the crucial factor in biasing of these events. We will refer to this conjecture as the "observational hypothesis on parapsychological phenomena" (OHPP).